And where is it? Someone said that there's a black box under the driver's seat and to pull the wires from it, but it's not there (it's my sister's car). She says that the alarm will go off at random times, and that she can start the car with the alarm going off, so she'd like to pull the plug on it.

I think i depends on the alarm itself. If it is a stock one then I have no idea. I know on my old car there is a button in my fuse box that was plced there when it wa installed. To disable my alarm I have to turn my key to the accesory position and hold the button till the alrm light flashes.